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14.8 - Concluding remarks

Integration of responses to temperature and other environmental factors

Considerable effort has gone into modelling relations between thermal environment and plant responses with a number of specific aims, namely to predict the likelihood of success of extending a particular crop into new regions, to predict phenology in a particular season and ensure the most effective application of fertiliser and agricultural chemicals, or to gain a better understanding of the basic processes that limit yield.

These models will often include data on maximum and minimum temperatures, solar radiation, photoperiod, rainfall, evaporative demand, soil water-holding capacity and nutrition. Temperature is thus one of several environmental factors that may influence plant growth and development and it is important to recognise the potential interaction between temperature and these other factors.

Light has an important role in regulating the effect of low temperature on the chlorophyll status of leaves of chilling-sensitive species and therefore on photosynthesis and growth. Interactions between temperature and light are complex and influence early stages of seedling establishment through to shoot number (branching), leaf shape and canopy development. In any crop, not all plant organs are at the same temperature, while direction, level and duration of incident radiation are also varying constantly. The combination of light and temperature vary from one location to another and although in a Mediterranean climate rising temperatures are often associated with increasing light, in the monsoonal regions of the tropics the high summer temperatures can be associated with high cloud cover and low light.

Temperature is also an important factor in plant water use. Low temperature can restrict the uptake of water by roots in some species, while high temperature, by lowering the relative humidity of the air (which increases the vapour pressure deficit), will increase the evaporative demand of the air and increase the rate of transpiration through the leaves. The latter will result in more rapid use of soil water and increase the possibility of drought. One of the main difficulties in assessing the interaction between temperature and water stress is because temperature influences both water use and the rate of plant development in parallel.

Uptake of mineral nutrients and their redistribution from one organ to another within plants are influenced by both nutrient availability and temperature. The optimum temperature for nutrient uptake varies between species and also from one mineral element to another. The importance of nutrition in relation to temperature then depends on whether the uptake and redistribution of nutrients can keep pace with the increased growth rates that are observed, for example, with increasing temperature. This would appear to be the case for phosphorus in wheat (even when the supply of phosphorus is low) where increasing temperature results in an increased concentration of leaf phosphorus, an increase that is also expressed in the grains at maturity. Thus in this example any deleterious effect of high temperature on yield would not appear to be mediated through plant phosphorus.

Summary

Vascular plants have come to occupy virtually every stable niche on earth during the course of their evolutionary history, regardless of thermal regime, and with remarkable adaptive capacity to acclimate to heat and cold. Plants may not necessarily thrive under extreme conditions, but they can survive, and are able to achieve a positive carbon balance and complete their life cycles due to physiological mechanisms and morphological features that lend thermal resilience.

Temperature extremes, especially in combination with other environmental stresses, impose an intense selection pressure. Cycles of vegetative growth and reproductive development have become closely attuned to such conditions, especially where growing seasons are brief and dormancy protracted. Such genotypes thus become highly specialised in their thermal responses.

Under more moderate conditions, survival mechanisms are of less importance and temperature assumes a different role in shaping genotypes by setting the biological tempo of ecosystems. Growth rate and reproductive effectiveness then become paramount, and again a genotype × environment interaction is apparent in the direction of biological responses due to temperature effects on carbon gain and reproductive development. An appreciation of processes underlying such responses lends a new dimension to our appreciation of natural ecosystems and our management options for communities of cultivated plants.